Lean burn operating engines utilize emission control devices coupled to the engine to store NOx while operating lean, and then to reduce stored NOx when the engine operates rich.
Typically, such systems utilized multiple catalysts in the exhaust to perform such a feature, where the downstream catalyst stores more NOx than an upstream catalyst while both the catalysts can store oxygen.
In such prior art approaches, it is desirable to minimize the number of times the engine transitions from lean to rich to purge stored NOx. This is because at each transition, the rich exhaust gasses must first reduce stored oxidants in the upstream catalyst before these gasses can reduce stored oxidants in the downstream. Thus, the present inventors have recognized that such a configuration of catalysts can result in inefficient rich (purge) cycles.
The inventors have also recognized that such a configuration is typically due to temperature constraints of the downstream catalyst. In other words, the catalyst storing more NOx is placed downstream so that it can operate in a lower temperature region, whereas the catalyst that stores more pure oxygen can typically operate in higher temperature regions and is therefore placed upstream. However, as discussed above, such a configuration results in wasting fuel to purge stored oxygen every time that it is desired to purge stored NOx.